The development of animal models to study human diseases has significantly advanced understanding of the underlying mechanisms of several diseases, including cancer. To date, animal models, particularly mice, have proven to be excellent candidates for the evaluation of the efficiency and efficacy of drugs and therapy options. While the utilization of these surrogate models to study human biology and diseases can be largely justified (due to ethical and technical constrains on the conduct of experimental therapies in humans) studies have highlighted potential limitations of extrapolating data from mice to humans (Mestas J, Hughes C C. (2004) Of mice and not men: differences between mouse and human immunology. J Immunol. 172:2731-2738).
To overcome these issues, there has been a long-standing interest in developing humanized mouse models. Intensive work by several groups have successfully demonstrated the feasibility of studying human biology and diseases in mice. Since having a functional and effective immune system in recipients will result in the elimination of the transplanted tissues/cells of human origin, using genetic mutants that lack cells of the adaptive immune system such as T, B and NK cells has significantly contributed to the success of the humanized mouse model. Accordingly, the most effective candidates of humanized mouse models include the NOD-SCID and the Balb/c strains that lack genes including recombination activating genes (RAG), common gamma chain (γC, also known as “interleukin 2 receptor, gamma”, or IL2rg), beta2 microglobin (B2M) and Perforin 1 (Prf1) (Shultz L D, et al. (2007) Humanized mice in translational biomedical research, Nat. Rev. Immunol. 7:118-130). Several studies over the past few decades have demonstrated the feasibility of transplanting several types of human tissues, including peripheral blood leukocytes, fetal liver cells, fetal bone, fetal thymus, fetal lymph nodes, vascularized skin, artery segments and either mobilized or cord blood hematopoietic stem cells (HSCs), into certain humanized mice (Macchiarini F., et al. (2005) Humanized mice: are we there yet? J. Exp. Med. 202:1307-1311). This approach is thought to provide better model systems since the data obtained from human cells in these mice might reflect the physiology of the human system. A major avenue of investigation in the field is to generate mice with a complete hematopoietic system and a functional immune system of the human origin. While significant progress has been made in generating immunocompromised mice with human T lymphocytes, B lymphocytes, NK cells and dendritic cells (DCs), there are still several challenges in the field, one of which is poor myeloid differentiation in the humanized mice.
Interestingly, there has been much progress in generating human T cells, B cells, NK cells and dendritic cells (DCs) from hematopoietic stem cells (HSCs) in humanized mice. In addition to the individual hematopoietic compartment, injection of human HSCs in these mice resulted in the reconstitution of lymphoid organs such as thymus and spleen. Nevertheless, the frequencies of myeloid cells, particularly granulocytes, macrophages, erythrocytes and megakaryocytes, are very low—a result that is probably due to inefficient myelopoiesis from human HSCs in these mice (Shultz et al. (2007); Macchiarini et al. (2005)). In view of the fact that the cells of myeloid origin (such as erythrocytes and megakaryocytes) are vital for the normal functioning of the blood system, and granulocytes and macrophages are critical for the development of the adaptive immune system, generating humanized mice with an efficient human myelopoiesis is of paramount importance.
Accordingly, there is a need in the art for genetically modified mice that are capable of improved human myelopoiesis upon engraftment with human HSCs (Manz M G. Human-hemato-lymphoid-system mice: opportunities and challenges. Immunity. 2007 May; 26(5):537-41).